Considerations about the pricing level
Generally, there are three parameters you should consider:
The performance you need from the service
Select the appropriate App Service plan for your App Service based on your needs. You can scale up or down the application. This should also influence your selection of the Azure Cognitive Search SKU.
Size and number of knowledge bases
Choose the appropriate Azure Search SKU for your scenario. Generally, you decide the amount of knowledge bases you need based on the amount of different topical domains. One domain of matter (for a single language) should be in one knowledge base.
Your Azure Search service resource must have been created after January 2019, and cannot be in the Free (Shared) tier. There is no support for setting customer-managed keys in the Azure portal.
For example, if your level has 15 allowed indices, you can publish 14 knowledge bases (one index per published knowledge base). The fifteenth index is used for all knowledge bases for creation and testing.
Number of documents as sources
The free QnA Maker management service SKU limits the number of documents that can be managed through the portal and APIs to 3 (each 1 MB). The Standard SKU has no limits on the amount of documents it can manage.